Fluorescent whiteners are commonly used by manufacturers to make a product brighter or whiter The fluorescent whiteners achieve the desired effect by responding to ultraviolet light and emitting visible light. There is a need to test the effect of the fluorescent whiteners, that is to determine how much additional light if any over and above the reflected visible light is emitted by the sample at different wavelengths in response to the ultraviolet radiation. One way to perform this test is to irradiate a sample with visible light, measure the resulting reflected light at different wavelengths by a spectrometer, then irradiate the sample with combined visible and ultraviolet energy, and then again measure the light given off by the sample at different wavelengths by the spectrometer.
The present invention provides an instrument which is designed to facilitate irradiating samples with visible light, near-infrared and ultraviolet energy and quickly make measurements of the resulting energy given off by the sample at different wavelengths throughout the visible and near-infrared spectrum. In accordance with the invention, a fiber optic probe is provided designed to irradiate the sample with visible, near-infrared (NIR) and ultraviolet light and to transmit visible and NIR light received back from the sample to the entrance slit of a spectrometer. The portion of the instrument for transmitting light to the sample contains glass fibers for transmitting visible and NIR light and quartz fibers for transmitting ultraviolet light. The transmitting ends of the quartz fibers and glass fibers are separated into a plurality of bundles which are distributed around an aperture in the probe to provide uniform illumination of a sample positioned over the aperture with both visible, NIR and ultraviolet light. The entrance ends of the glass fibers are arranged to receive light from a source of visible and NIR light, and the entrance ends of the quartz fibers are arranged to receive light from an ultraviolet source.
The spectrometer is provided with a fixed grating and a fixed array of photodetectors positioned to receive light dispersed into a spectrum from the fixed grating. Each photodetector of the array is positioned to detect the light from a different narrow bandwidth from the grating. The output signals of the photodetectors are amplified by amplifiers mounted within the housing of the spectrometer and the amplified signals are applied to a computer system which processes the data represented by the amplified output signals.
The probe has mounted therein a standard white sample, which is pivotable under the control of the computer system to a position in the path of the light transmitted by the fiber optics. To calibrate the instrument, the standard white sample is pivoted into position and the amplitudes of the resulting output signals from the amplifiers are stored in the computer. From these stored amplitudes, a calibration factor is computed for each photodetector and amplifier combination. This calibration factor, when multiplied by each amplified output signal would provide the same value for each photodetector if a perfect white sample were positioned over the probe aperture. These calibration factors are then used to multiply the amplified output signals from the photodetectors when a sample to be tested or measured is in position over the probe to provide the data representing the spectral energy distribution and fluorescence characteristic of the test sample.
In accordance with the invention, one or more temperature transducers are mounted in the spectrometer housing to sense the temperature within the spectrometer housing. When such a temperature transducer detects a predetermined small change in temperature in the spectrometer housing, 0.1.degree. C. for example, the white standard sample is automatically pivoted into position and the calibration factors are recalculated. Because both the amplifiers and the photodetectors are mounted in the spectrometer housing where the temperature transducer or transducers are mounted, the automatic calibration avoids error due to temperature change for both the photodectors and the amplifiers.
In addition to automatically recalculating calibration factors whenever the temperature changes within the spectrometer housing, the computer system will also cause a recalibration to be periodically carried out at preselected time intervals.
In addition to this recalibration, the system may be operated in a mode in which at relatively short time intervals, the standard white sample is pivoted into position and the ratio of the amplified output signal from a photodetector near one end of the array to the amplified output from a photodetector near the other end of the array is determined. When this ratio changes by more than a predetermined small amount for example, 0.1 percent, the calibration factors are recalculated.
In the manufacture of the instrument, the photodetector are mounted in the spectrometer housing to be slideably movable by means of a micrometer screw in a direction to adjust the bandwidth received by each photodector. When this adjustment has been made, screws retaining the detector assembly are potted in position so that the position of the photodetectors is permanently precisely fixed in the proper position. The micrometer screw is then removed.
To use the instrument, the probe is positioned to irradiate a sample and the visible and NIR light source is energized to irradiate a sample with visible and NIR light. Measurements are then made with the array of photodetectors in the spectrometer. Next, the source of ultraviolet light is energized so that the sample is irradiated with visible, NIR and ultraviolet light and measurements are again made with the photodetectors. The difference in the results from each photodetector in the array will provide an indication of the effect of the fluorescence, if any, of the sample at the narrow bandwidth detected by the photodetector.
Because the probe is implemented by fiber optics, it can be readily applied to a large number of successive samples in rapid succession. Because the photodetectors are a fixed array, the fluorescence and reflectance from the successive samples can be measured and analyzed quickly, thus permitting measurement of samples in rapid succession.
Further advantages of the present invention will become readily apparent from the following detailed description of the invention when considered in conjunction with the drawings.